1. Field of the Invention
This invention relates to a process for producing alkali metal hypochlorites, especially sodium hypochlorite, and an electrolytic cell suitable for this process.
2. Description of the Prior Art
Alkali metal hypochlorites, especially sodium hypochlorite, are used as oxidizing agents for oxidizing treatment of water in order to sterilize, decolorize, and deodorize river water, city water, sewage water, waste water, etc., for iron elimination, and for manganese elimination of waste water from industrial establishments. When commercially available sodium hypochlorite is used, an aqueous solution of sodium hypochlorite usually having a concentration of 10 to 12% by weight must be transported to a water-treatment facility. Thus, about 7 to 9 times by weight of water to the sodium hypochlorite must be transported simultaneously. The cost of transportation is thus high, and moreover, the sodium hypochlorite partly decomposes or forms a chlorate due to the effect of temperature, impurities, sunlight, etc. during storage, and the concentration of available chlorine decreases. The reduction of the concentration of available chlorine is especially marked in the summertime, and generally about 25% of the available chlorine is lost.
To avoid this disadvantage, it is desirable to provide a non-diaphragm electrolytic cell at a site adjacent a water-treatment facility, and to produce an aqueous solution of an alkali metal hypochlorite efficiently in situ by electrolysis of an aqueous solution of an alkali metal chloride for use in water treatment or other purposes.
U.S. Pat. No. 3,849,281 is a recent approach to providing a means for on-site generation of alkali metal hypochlorite solutions and discloses a substantially vertical electrolytic cell containing a plurality of unit cells separated by horizontal partitions between the unit cells for electrolysis of alkali metal chloride aqueous solutions. However, in operation of the electrolytic cell described in U.S. Pat. No. 3,849,281, gas generated at the cathode in each unit cell contacts the cathode in the next adjacent unit cell, because cathodes are present in a passageway through which gas rises in the unit cells. This causes a marked increase of the electrical resistance of the electrolyte between the anodes and the cathodes and reduces the current efficiency of the electrolytic cell.
Thus, it is further desirable to reduce the operating cost suitably for water treatment by performing the electrolysis at high current efficiencies. In other words, an electrolytic cell which yields a large voltage with a small current capacity (one comprising a plurality of unit cells arranged in series) is preferred to increase the power efficiency (i.e., the conversion efficiency required for transformation and rectification) and reduce the cost of the equipment. For the unit cells, means to reduce the voltage of each unit cell and increase the current efficiency in each unit cell in an electrolytic cell must be provided.